The Firing Process: Understanding the Basics to Use a Kiln

Two Main Firing Stages

First, bisque firing transforms the greenware into a durable, porous material. The semi-vitrified state is where you can safely glaze and decorate ceramics. Carbonaceous materials are burned from the clay during the stage as well.

As the name suggests (glaze firing), the second firing stage takes place after you add glaze to your ceramic piece. The glaze firing stage is often faster than bisque firing because the clay already contains less water. Its purpose is to mold the glaze onto the pottery.

How a Kiln Transfers Heat

Electric kilns all come with heating elements in the form of a piece of coiled wire that resists electricity from passing through, like a stovetop or any other home heating appliance.

The coiled heating element in an electric kiln keeps electrical movement from taking place while allowing the wire to radiate heat throughout the interior of the kiln. Heat rises inside the kiln before it’s absorbed by whatever is inside the kiln (your ceramics).

Pyrometric Cones

According to Ceramic Arts Network, pyrometric cones are necessary when firing ceramic art with high-tech computerized kilns. However, you don’t need pyrometric cones if your kiln is controlled automatically.

Cones measure the amount of heat your ceramics absorb inside the kiln, where the higher cone number equals the higher the internal temperature you can fire.

The lower cone temperatures display a zero before the number. For example, you may see the common low cone numbers 018, 019, and 020. Higher cone temperatures appear as two-digit numbers, such as 12, 13, or 14. You can check out a full pyrometric cone chart from the Ceramics Shop.

Traditionally, you use a three-cone system each time you fire. The first cone is a guide with a number just below your target firing temperature. The second cone is the firing cone, which is the target temperature. Finally, the guard cone is a single number above your target firing temperature.

When nearing the end of the cone’s peak temperature range, it may soften near the tip and bend slightly. These cues tell you when you’re firing at the intended temperature. Thus, using pyrometric cones provides immediate feedback and helps you ensure the pottery fires correctly.

Even if you have an automatic kiln, the three-cone system is suggested for no less than every three firings to make sure your kiln’s temperature is calibrated correctly. Hot or cold spots in a kiln could cause trouble, and cones allow you to troubleshoot other inconsistency issues like with the glaze color.

Troubleshooting your kiln with pyrometric cones is especially helpful with older kilns, as the heating element can become less reliable over time.

Before You Start

When you’re ready for firing, double-check everything. Consider the following important information to ensure your pottery doesn’t crack.

Invest in Kiln Furniture

A wide range of accessories and furniture are helpful with electric kilns. Some accessories support clay objects during the firing process, while furniture created with kilns in mind can withstand high temperatures without deforming.

Consider purchasing the following accessories:

Kiln shelves: Cordierite is the most common material used for kiln shelves, which is a natural mineral with silica and alumina. It can withstand repeated firing at cone 10 but is less likely to warp at temperatures below cone 8. High-alumina kiln shelves are better for higher temperatures, as they can handle up to cone 11 with ease. The last option, silicon carbide, is thinner and more lightweight. This shelf won’t warp at high temperatures but can cost nearly twice as much.

Kiln posts: A tool for kiln shelves, posts support the shelves while optimizing your firing space. There are many different options you can purchase in varying thickness levels and heights.

Kiln stilts: Use stilts to hold and protect pieces in low or medium temperature firings. Some stilts can maintain their form up to cone 10, but most are better for temperatures below cone 6.

Bricks: Line bricks around the inside of a kiln for insulation. Hard bricks are more strong and dense, making them excellent for adding structural support. Soft options are less capable in high temperatures but tend to retain heat very well.

Furniture kits: Kits often come with everything, such as kiln shelves, posts in various sizes, heat resistant gloves, and cleaning equipment. Dozens of options are available, depending on your kiln’s needs. They’re the easiest way to get all the accessories you need simultaneously.

Keep the Kiln Clean and Ready

Make sure your kiln is clean and in top condition. Regularly check the heating elements for damage, ensure the lid braces are secure, and check out the electrical cords.

When cleaning the kiln, vacuum inside. You may need to chisel off any clay or glaze drops from the shelves as well. Firebrick kilns especially require regular and thorough cleaning. If the heating element is exposed to foreign matter, they may short out.

You should also double-check that there are no combustible materials near the surrounding area. Keep no less than a foot between your kiln and combustibles at all times for safety.

Determine the Right Temperature

Consider what temperature is ideal for firing your clay and glaze. All options mature at specific and varying temperatures.

If you fire the clay too hot, the ceramics can deform or completely melt. Too low a temperature results in unsolidified ceramics that are not durable. Firing a glaze at too high a temperature can cause it to runoff, whereas temperatures that are too low result in dry, rough pieces. The color can also come out wrong with variance in temperature as well.

How to Use a Kiln: The Two Firing Stages

When you have everything ready to fire, use the following steps for a safe and successful process. The basic rules you follow vary based on if you’re firing using the bisque method or glazing your piece.

The Bisque Load

The first stage to fire ceramics is bisque firing, and you should try to fire a full load to take advantage of the heat and save the amount of electricity you use each time. Handle the fragile work with care, and ensure your pieces are bone dry before firing. The clay should have no water left, or the steam may cause your ceramics to explode or break.

Start out using the candling technique to dry your damp clay. There are other ways you can dry pottery and clay objects to prepare them for firing as well. In the meantime, turn the kiln on to a low temperature of 180 degrees F for eight to ten hours. Preheating the kiln eliminates the chances of breakage later.

You can bisque fire between cones 010 and 04, where 08 to 06 are the most common options. To choose which cones are best for your project, select either a low of high fire bisque load method.

Low fire bisque firing involves higher-numbered cones up to 04 to burn out all the carbon and other materials from the clay during the first firing. Removing the carbon now keeps it from later burning out during the glazing process, which can cause blisters. The pieces also come out stronger and are less likely to crack while cooling.

High fire, on the other hand, comes with more risk. The clay may not absorb enough glaze, resulting in less porous and flexible pieces that are sensitive to thermal shock. This process involves bisquing close to cones 010, and the resulting ceramics become porous and absorb glaze well.

The problem with hire firing is that the piece can actually absorb too much glaze, causing it to become too thick and fall apart. Firing near a cone 010 results in weaker pieces as well. Try to choose the cone number based on your glaze and what you’re making for this reason.

Slow firing is safer than a fast process, so only use fast-firing if that helps your glaze look better. Otherwise, a slower process will help make sure the ceramics don’t crack under quickly rising temperatures.

For the best results, place the uncooked piece on a shelf using one-inch stilts to hold the bottom shelf and add air circulation inside the kiln. Set each piece at least an inch away from the heating elements, walls, and the thermocouple.

Then, make sure the steam can escape from the kiln or the pieces may explode. Prop the lid open a few inches using a brick for the first few hours if you don’t have a proper venting system. Unplug the upper peephole of your kiln for the first few hours as well.

Heat the pieces for as long as necessary, then turn off the kiln. Even unplugged, the kiln remains hot for a long time. Avoid touching it (or the ceramics inside) until it fully cools. Only unload the kiln when you can easily touch your pieces by hand.

Glaze Firing

Glaze firing involves many of the same rules as bisque firing, like the ventilation requirement during the first few hours of firing and the tendency to perform better at slow speeds. However, there are also a few vital differences to keep in mind.

This second firing takes less time than bisque firing because most of the moisture in the clay is already removed. Some glazes may appear better with slow firing, while others prefer a fast process. If you’re not sure, always start slow.

Experiment to find the best option for your needs.

Common commercial glazes require firing with two cones cooler than the cone you used during bisque firing. However, many potters use the same cone for both firing steps with low-fire pieces. The most common cone for glaze firing is 06, but the best cone for you may vary based on the:

Project

Glaze

Kiln

Use a hold or soak temperature at the end of glaze firing to keep the heat inside even and assure all the pieces are complete. This step is vital if the kiln is full. Although, avoid leaving them to soak too long or you may overfire the pieces.

If you’re using a manual kiln, alternatively, you’ll want to switch the kiln to medium heat rather than turn it off. A kiln with an automatic controller also allows you to program the kiln to cool on a schedule.

Kiln Operation Safety

A kiln reaches super high temperatures that can cause injury. They also potentially release dangerous gases during the firing process, which can pose many hazards you must avoid. Keep the following crucial safety rules in mind before, during, and after firing.

Before

To keep yourself safe, complete the following before firing:

Buy personal protective gear like kiln mitts and dark glasses

Read your kiln’s manual and learn to safely install and operate it

Set up the ventilation system for harmful fumes and gasses

Double-check the shelves are sturdy and uncracked

Keep the kiln unplugged and turned off before loading it

During

While the pottery is firing, don’t:

Touch the heating elements with anything

Leave the kiln unattended

Look inside with safety glasses

Wear loose-fitting clothing

Add insulation around the kiln or the wiring may overheat

If you smell burning plastic, immediately turn off the kiln. Examine the cord and wall outlet for burning signs.

After

When the kiln is still hot, turn it off and unplug it. Don’t:

Open the kiln until it reaches room temperature

Lift the lid when it’s not in use

Place anything on top of the kiln lid

Allow pets or children near the kiln

Experiment with Care

Now that you understand how to use your electric kiln safely, enjoy it! Experiment to figure out which techniques work best for your project. Use the above tips to properly use the kiln, fire ceramics at home safely, and ensure fewer mistakes each time.

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